Froth flotation is a pre-concentration process extensively used in processing low-grade ores. Effective pre-concentration operations concentrate most of the valuable mineral into a small mass and thus reduce equipment sizes, chemical consumption and material handling in subsequent stages. In the current industrial practice of concentrating lead-zinc ores using froth flotation, first a lead mineral (galena) is pre-concentrated in stage-1 as lead-rougher (Pb-R) and then in the subsequent stage (Stage-2) the zinc mineral (sphalerite) is concentrated as the zinc-rougher (Zn-R). In order to avoid any sphalerite coming into the Pb-R stage, it is suppressed by adding reagents such as sodium metabisulfite (MBS), sodium cyanide and zinc sulfate. Galena, the lead mineral, is floated using potassium ethyl xanthate (PEX) as the collector. After floating galena, sphalerite suppressed in the Pb-R stage is activated by copper sulfate and then floated using potassium amyl xanthate.
Copper sulfate not only activates the sphalerite suppressed in the Pb-R stage but also facilitates the formation of a more stable surface complex with the xanthate collector, thereby enabling its flotation into the froth.
The amount of copper sulfate added depends on the grade of sphalerite in the ore. Generally, about 1 kg of copper sulfate is added per tonne of ore. Hence, a mill processing about 20,000 tonnes of ore per day requires 20 tonnes of copper sulfate. It is of note that zinc in minerals does not form a stable surface complex with xanthates. Consequently, xanthates by themselves are not capable of floating the zinc minerals effectively.
Copper ions in copper sulfate added for activation attach to the zinc sites and form a stable surface complex with the xanthate collector and thus, copper sulfate performs a dual action of activating suppressed sphalerite and forming a more stable surface complex with xanthate.
Generic structure of xanthates.
It is very important to keep the pyrite (the gangue mineral) in the tailings rather than co-floating into the float concentrate. Pyrite contamination in the zinc float concentrate reduces the value of the concentrate and renders it commercially less profitable.
Among the auxiliary chemicals used in a flotation circuit, copper sulfate is the most expensive and corrosive. The corrosive nature of copper sulfate reduces the useful life of machinery. Also, the presence of copper makes mill effluents toxic, meaning that expensive effluent-treatment is needed before discharge. Furthermore, the shelf-life of xanthates is limited due to their decomposition in moist environments, and auto-decomposition of xanthates produces harmful volatile substances such as carbon disulfide. Finally, the copper sulfate is unrecoverable for re-use or recycling and ends up in the effluents.